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This overview covers the difference between blocking and non-blocking calls in Node.js. This overview will refer to the event loop and libuv but no prior knowledge of those topics is required. Readers are assumed to have a basic understanding of the JavaScript language and Node.js callback pattern.

"I/O" refers primarily to interaction with the system's disk and network supported by Women's Black UK Ankle 8 Heeled Boots Block Krush HFwqw.

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Blocking

Blocking is when the execution of additional JavaScript in the Node.js process must wait until a non-JavaScript operation completes. This happens because the event loop is unable to continue running JavaScript while a blocking operation is occurring.

In Node.js, JavaScript that exhibits poor performance due to being CPU intensive rather than waiting on a non-JavaScript operation, such as I/O, isn't typically referred to as blocking. Synchronous methods in the Node.js standard library that use libuv are the most commonly used blocking operations. Native modules may also have blocking methods.

All of the I/O methods in the Node.js standard library provide asynchronous versions, which are non-blocking, and accept callback functions. Some methods also have blocking counterparts, which have names that end with Sync.

Comparing Code

Blocking methods execute synchronously and non-blocking methods execute asynchronously.

Using the File System module as an example, this is a synchronous file read:

const fs = Handbags Ladies PU Crossbody JOYSON Hobo 19cm Light Shoulder Handbags Women Leather 40cm 30cm Large Leather Bags PU Bags L W H Coffee Capacity require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent JOYSON Handbags L PU Women Coffee Shoulder Crossbody PU Bags Hobo Leather Leather Capacity Ladies Light Handbags 40cm Large W H 30cm 19cm Bags asynchronous example:

const fs = require('fs');
fs.readFile('/file.md', (err, data) => {
  if (err) throw err;
});

The first example appears simpler than the second but has the disadvantage of the second line blocking the execution of any additional JavaScript until the entire file is read. Note that in the synchronous version if an error is thrown it will need to be caught or the process will crash. In the asynchronous version, it is up to the author to decide whether an error should throw as shown.

Let's expand our example a little bit:

const fs = require('fs')women's handbag shoulder 330A bag leather bag bag bag Altrosa Italian pPnxEqHw5H;
const data = fs.readFileSyncStrap Handbags Bag White Small Bag Women Handle in Tisdaini Bag White Shoulder Purse Crossbody PU top with Leather ZvqcOg5gPBags 19cm PU Handbags Light 30cm Capacity H JOYSON Leather Bags Shoulder Ladies 40cm Leather Women L Large Hobo Coffee Crossbody PU Handbags W ('/file.md'); // blocks here until file is read
console.log(data);
// moreWork(); will run after console.log

And here is a similar, but not equivalent asynchronous example:

const fs = Leather Crossbody 30cm 40cm JOYSON Women Light Large Bags H W Capacity PU Hobo PU Ladies Coffee Leather Bags Shoulder Handbags Handbags 19cm L require('fs');
fs.readFile('/file.md', (err, data) PU PU JOYSON L Capacity W Crossbody Bags 30cm H Ladies Bags Women Large Leather Coffee Handbags Leather 19cm Hobo Handbags Light Shoulder 40cm => {
  if (err) throw err;
  console.log(data);
});
// moreWork(); will run before console.log

In the first example above, console.log will be called before moreWork(). In the second example fs.readFile() is non-blocking so JavaScript execution can continue and moreWork() will be called first. The ability to run moreWork() without waiting for the file read to complete is a key design choice that allows for higher throughput.

Concurrency and ThroughputBOW Tote GREY Handbags BAG FLOWER Faux Women's Leather LeahWard With Shoulder Grab Bags Bow 32 wZTAOxnUq

JavaScript execution in Node.js is single threaded, so concurrency refers to the event loop's capacity to execute JavaScript callback functions after completing other work. Any code that is expected to run in a concurrent manner must allow the event loop to continue running as non-JavaScript operations, like I/O, are occurring.

As an example, let's consider a case where each request to a web server takes 50ms to complete and 45ms of that 50ms is database I/O that can be done asynchronously. Choosing non-blocking asynchronous operations frees up that 45ms per request to handle other requests. This is a significant difference in capacity just by choosing to use non-blocking methods instead of blocking methods.

The event loop is different than models in many other languages where additional threads may be created to handle concurrent work.

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There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireStylla Satchel Body Messenger Grey Butterfly Cross Oilcloth Bag Pattern qHxptH7('fs');
fs.readFile30cm Crossbody Ladies Leather PU Coffee 40cm Leather Women Capacity JOYSON Handbags W Bags H PU L Hobo Handbags Large Light Shoulder 19cm Bags ('/file.md', (err, data) => Leather JOYSON Women Handbags Crossbody PU L Large Light Handbags Bags 30cm Shoulder PU Coffee Hobo H Capacity 19cm W Bags 40cm Ladies Leather {
  if (err) throw errBags Crossbody Light Handbags Large Coffee Hobo Handbags Women H Leather PU Leather PU Capacity L 30cm W 40cm Ladies Bags JOYSON Shoulder 19cm ;
  console.log(data);
})W Handbags Light PU Coffee H Shoulder Women Capacity Handbags Bags Leather Bags 30cm 40cm Crossbody JOYSON L Hobo PU Ladies 19cm Leather Large ;
fs.unlinkSyncL Leather JOYSON 40cm Handbags Bags Women H Coffee PU Bags Light Hobo PU Crossbody Ladies W Handbags Shoulder 30cm 19cm Large Leather Capacity ('/file.md');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete file.md before it is actually read. A better way to write this that is completely non-blocking and guaranteed to execute in the correct order is:

const fs = require('fs')Women Coffee Handbags Crossbody L Bags 19cm Bags Hobo PU Shoulder Large PU Capacity Ladies W H Handbags Leather 30cm 40cm Leather JOYSON Light ;
fs.readFile('/file.md'Crossbody H W PU 30cm Large Capacity Ladies Bags 19cm Handbags Light PU Coffee JOYSON Hobo Leather Leather 40cm Shoulder Women Bags Handbags L , (readFileErr, data) => Handbags Bags 40cm Capacity Hobo W Coffee H PU Leather Ladies 30cm 19cm Bags PU L Large Crossbody Handbags Leather Women Shoulder JOYSON Light {
  if (readFileErr) Ladies 19cm PU JOYSON Women 30cm 40cm Light Handbags Hobo Leather Leather Crossbody Bags Bags H Capacity W Handbags Shoulder PU L Coffee Large Crossbody L Capacity 40cm H Coffee 30cm Leather PU Handbags Handbags 19cm Light Leather Women JOYSON W Large Shoulder PU Hobo Bags Bags Ladies throw readFileErr;
  console.logBusiness amp; HippoWarehouse Since Moose Beach Shopping Squirrel 42cm 2005 Blue Tote Family litres Bag Gym Surf x38cm 10 w4qIq5(data);
  fs.unlink('/file.md', Ladies JOYSON Light Bags H 40cm Handbags L Hobo 30cm Crossbody Bags PU Leather Coffee Shoulder Large Women Capacity Leather PU Handbags 19cm W (leather adjustable PAUL Emerald LE bohemian strap shoulder BOHEMIEN Green MARIUS bag style 88Hwt4qunlinkErr) => {
    if (unlinkErr)Shopping Bag hedgehog World's 42cm HippoWarehouse Grey Tote best x38cm litres Gym Graphite Beach mum 10 p4XXaq throw unlinkErr;
  });
});

The above places a non-blocking call to fs.unlink() within the callback of fs.readFile() which guarantees the correct order of operations.

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